There has been well-known heat-sensitive recording materials utilizing the colorforming reaction between a colorless or pale colored basic chromogenic material and an organic or inorganic color developer, in which color images are produced by heating to contact with each other of the basic chromogenic material and the color developer.
These heat-sensitive recording materials are relatively cheap and the recording machines are compact and easily maintained. Therefore, they have been used in the various fields. However, when the heat-sensitive recording materials are generally allowed to stand in an atmosphere of about 40.degree. C. for a long time, for example in the course of transportation or in the storehouse, undesired colorforming reaction (fogging) is partially occurred. Particularly in high-sensitive heat-sensitive recording materials, the brightness of the background area is remarkably lowered. Therefore, it has been strongly desired to develop heat-sensitive recording materials in which the brightness is not substantially lowered by exposing to high temperature condition for a long time.
Generally, heat-sensitive recording materials do not have sufficient retainability of the recorded images. Particularly the recorded images are discolored by exposing to high temperature condition for a relatively short time. To improve the image retainability, there have been proposed various methods in which an agent for improving the image retainability is used. However, with use of the conventional agents for improving the image retainability, a sufficient effect is not obtained, or a new problem such as a lowering of brightness of the background area occurs even if an agent having an effect for the retainability of the recorded images at high temperature is used, the desired results can not be always obtained.
Therefore, it has been strongly desired to provide a heat-sensitive recording material in which the background area is less susceptible to a lowering of brightness by storing under high temperature condition and the recorded images are stably maintained.
The object of the present invention is to provide a heat-sensitive recording material in which both of the brightness of the background area and the optical density of the recorded images are stably maintained under high temperature condition.